Which of the following nitrogenous bases is present in RNA but completely absent in DNA?
Answer and explanation
Correct answer: B. Uracil
- Step 1: Nucleic acids contain two classes of nitrogenous bases: purines (adenine, guanine) and pyrimidines (cytosine, thymine, uracil).
- Step 2: DNA contains the four bases adenine (A), guanine (G), cytosine (C), and thymine (T).
- Step 3: In RNA, thymine is replaced by uracil (U), which lacks the 5-methyl group present on thymine.
Answer
Uracil.
Why not the others: Thymine is present in DNA but absent in RNA; Cytosine is present in both DNA and RNA; Adenine is present in both DNA and RNA.
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